Process for removing impurities from the precipitation system of the bayer process



Mamh 1968 R. F. ROBERTS, JR. ETAL 3,372,985

PROCESS FOR REMOVING IMPURITIES FROM THE PRECIPITATION SYSTEM OF THEBAYER PROCESS Original Filed Feb. 20, 1963 5 Sheets-Sheet 2 mozteumma 9.8486mm Him 5 S E c En. m llllllllt'l P wk 4 xo zoi tn=ummm \lllll mZOFDJOm u xO w 2526 a on w o R T s mv D S E Q m 0 8mm 55% 2596 m m oziwSum 0252; 5mm R o R t Rm T O o m E T a m Y m m R R P 5236.5 235 5520;;EsEwR m ozouum odmm o 4 o c-m o o Em o R o m U Q D u Em T m mm E CG 9 mM HHH PREGNANT LIQUOR MR O u 5 TT R R A VBTJ N 5 F HM wTM ARI- mw GOW Rm March 12, 1968 R. F. ROBERTS, JR.. ETAL 3,3 ,98

PROCESS FOR REMOVING IMPURITIES FROM THE PRECIPITATION SYSTEM OF THEBAYER PROCESS 1965 5 Sheets-Sheet 5 Original Filed Feb. 20,

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PROCESS FOR REMOVING IMPURITIES FROM THE PRECIPITATION SYSTEM OF THEBAYER PROCESS Original Filed Feb. 20, 1963 5 Sheets-Sheet 5 96 FINESCYCLE O UNWASHED SEED 0 WASHED SEED INVENTORS REGINALD E ROBERTS JR.ROBERT H. BY WILLIAM A. JAMES United States Patent 3,372,985 PROCESS FORREMOVING IMPURITIES FROM THE PRECIPITATION SYSTEM OF THE BAYER PROCESSReginald F. Roberts, In, and Robert H. Lister, Baton Rouge, La., andWilliam A. James, Toronto, Ontario, Canada, assignors to Kaiser Aluminumand Chemical Corporation, Oakland, Calif., a corporation of DelawareContinuation of application Ser. No. 259,826, Feb. 20, 1963. Thisapplication June 5, 1967, Ser. No. 645,544 Claims. (Cl. 23143) Thisinvention relates to a process for the removal of water solublecompounds, such as oxalate salts, from the precipitation system of theBayer process. More particularly, the invention relates to a process forthe removal of oxalate contaminant from precipitated alumina hydrateprior to recycling the hydrate as seed for the precipitation system.Further, the invention also presents a method for the rectification orconversion of the removed oxalate for the recovery of caustic.

This application is a continuation of Ser. No. 259,826 filed Feb. 20,1963 and now abandoned.

The precipitation of alumina from clarified caustic aluminate liquorscan be accomplished in both batch and continuous precipitation systems.In batch precipitation, pregnant liquor is passed to a large tank, theliquor is seeded and precipitation continues in the same tank for apredetermined period of time with agitation of the slurry duringprecipitation. The contents of the tank are then removed and theprecipitated alumina hydrate is removed from the liquor. The liquorremaining, referred to as spent liquor, is recycled to the digestionsteps of the lBayer process after reconcentrating and addition of makeupcaustic. The alumina hydrate recovered is classified into product andseed. In a continuous system, pregnant liquor is treated in a series ofinterconnected tanks through which the pregnant liquor flows. In eachtank, precipitation of alumina hydrate is taking place. Examples ofcontinuous precipitation systems are those shown by Cowles in Patent2,606,820.

The manner in which seed is added to the pregnant liquor and thecharacter of the seed has a definite effect upon the particle size andsettling characteristics of the product. In both batch and continuoussystems, the practice is to charge the seed into the precipitators as aslurry or suspension in caustic liquor, for example, spent causticliquor. This is the condition in which the classified fine hydrate isnormally recovered from the classification system. This method gives arelatively fine and slow settling precipitation product.

The caustic liquor used as the medium to carry the seed to the pregnantliquor is liquor which has passed through the precipitation andclassification stages, and usually has had about half of the aluminaprecipitated therefrom. The bulk of the spent liquor is returned to theinitial digestion step after replenishing amounts of caustic have beenadded thereto, and contains various types of organic matter introducedinto the liquor stream during processing.

\ 'Most of this organic matter is a complex mixture of solubilized ordissolved organic substances, including salts, which are formed in thecaustic liquor from the organic matter, such as the humus, present inthe aluminous ore, and from starch which is employed as a flocculatingagent in removing the red mud residue of the ore from the liquor. Theprocess liquor is thus contaminated with these organic substances. Theseorganic materials are present in various forms from complex highmolecular weight compounds to more or less ultimate products, since theyare molecularly degraded by the caustic and Patent 1,943,786 and Harmsin high temperatures employed in the process to such lower molecularweight compounds as oxalates, glycolates, acetates, and inorganiccarbonates. In the usual practice of the Bayer process, caustic soda isused for bauxite digestion; and in this instance, the aforementionedcompounds would be sodium compounds. However, other alkali materials orcaustics, such as potassium hydroxide, could be used for bauxitedigestion and the instant invention also pertains to the organicsubstances formed in these systems.

The more or less ultimately degraded organic materials in solution areto be collectively referred to hereinaiter, in specification and in theclaims, as oxalate or sodium oxalate, since this fraction of organicmaterials in the liquor (as determined by a potentiometric titrationwith perchlorato-cerate in perchloric acid solution) is collectivelyreported as sodium oxalate. In addition, this sodium oxalate fraction ofthe organic matter in the liquor is the one with which the presentinvention is mainly concerned. It is to be understood, however, that theterm is inclusive of other forms of organic matter content in the liquorhaving an effect similar to the oxalate, whether identified or not. Thesodium oxalate is precipitated from the liquor as very fine particles atthe temperatures and caustic soda concentrations of the liquor in thealumina precipitation phase. They provide active surface sites for thenucleation of alumina hydrate particles and/or retard the agglomerationor growth of alumina hydrate particles thereby resulting in anundesirably fine and slowly settling alumina hydrate precipitate.

According to the instant invention sodium oxalate is removed from theseed fractions, usually the finer fractions, of the alumina hydrateparticles prior to using these particles as seed in the precipitationreaction. The alumina hydrate particles are subjected to a water washing to dissolve substantially all the sodium oxalate present. Theremoval of the sodium oxalate contaminant from the seed fraction aluminahydrate promotes better particle size control in the precipitation,increased capacity of the precipitation system, more efiicient operationof the classification system, a purer seed hydrate, and a purer productalumina. The invention also provides for treating the oxalate-richwashings to precipitate oxalate which can be further processed and torecover caustic soda which may be recycled in the process.

Other features and advantages will become apparent from the ensuingdisclosure.

It has been discovered that under the conditions present during theprecipitation phase of the Bayer process, the liquor is supersaturatedwith respect to sodium oxalate as Well as dissolved alumina. The oxalatesolids present in the seed promote the formation of a finely dividedalumina hydrate precipitate, and this problem is particularly acute inthe processing of bauxites having a high organic carbon content, such asJamaican bauxites.

As a consequence, the classification operation for separatin the aluminahydrate particles from precipitation into size fractions reveals thatthe finer fractions are high in sodium oxalate whereas the coarserfractions are substantially lower in sodium oxalate. This association ofthe sodium oxalate crystals with the finer fractions of the aluminahydrate permits the removal of the major part of the solid sodiumoxalate by treating only a portion of the alumina hydrate particles,namely, the finer fractions, which fractions are used as seed.

In the American Bayer process practice, the alumina precipitation phaseis normally operated batchwise with the holding time varying for any onebatch operation according to the demands of the plant. Usually, however,the retention time is about 25 to 55 hours. On the other hand, thehydrate classification system .is normally con- 5. spent caustic liquor,the primary filtrate, which can be recycled in the process.(Alternatively, a centrifuge could be employed in place of the rotarybelt filter 70.) The filter cake from filter 70 is subjected toagitation in dissolving or leaching tank 71 with hot water, preferablyat a temperature of 50 to 90 C., to dissolve the oxalate. The resultingslurry is then filtered on a second rotary belt type filter 72 afterwhich the filter cake is again washed with water to remove substantiallyall of the dissolved oxalate. The washed seed is fed to a repulperwherein the seed is reslurried with liquor and returned toprecipitation. The filtrate from filter 72 is oxalate-rich and can bediscarded or suitably treated, (for example, by adding caustic soda toprecipitate sodium oxalate, or lime to precipitate calcium oxalate withthe recovery of caustic, or by evaporation to precipitate sodium oxalateas previously discussed).

In extensive testing of washing, tertiary (tray) thickener hydrate in asystem such as that depicted in FIG- URE 3, it has been found thatalumina hydrate particles containing 0.4 to 6.5% by Weight of sodiumoxalate can be washed to yield a washed alumina seed having only to0.25% by weight of sodium oxalate. The secondary filtrate from filter 72will contain on the order of 6-32 g./l. sodium oxalate.

FIGURE 4 shows diagrammatically a feed washing system and procedure forwashing the alumina hydrate particles in the secondary thickener (ST)underflow. The underflow from the secondary thickener is fed to a rotarybelt filter 76 wherein the spent liquor is removed from the ST seeduntil the cake contains approximately 80% solids by weight. A waterspray washes the filter cake with water, preferably cold water, e.g.,less than 32 C. The amount of water used is preferably just sufficientto remove substantially all the spent caustic liquor, the primaryfiltrate which can be recycled in the process. The filter cake fromfilter 76 is subjected to agitation in dissolving or leaching tank 77with hot Water, preferably at a temperature of 50 to 90 C. to dissolvethe oxalate. The resulting slurry is then filtered on a second rotarybelt-type filter 78 after which the filter cake is washed with water.The washed seed is fed to a repulper 79 wherein the seed is reslurriedwith liquor and returned to precipitation. The filtrate from filter 78con tains the oxalate and can be discarded or suitably treated assimilarly described for FIGURE 3. In some instances of operation, thefiltrate from filter 78 is not sufiiciently high in oxalate to warranttreatment to precipitate the oxalate. The filtrate may, if desired, berecycled to the mud washing system or to the starch preparation systemin the Bayer process.

In extensive washing tests of secondary thickener hydrate in a systemsuch as that depicted in FIGURE 4, it has been found that with secondarythickener hydrate containing 0.1 to 2.0 weight percent of sodium oxalatethe sodium oxalate can be reduced on the Washed hydrate to 0 to 1% byweight. The secondary filtrate from filter 78 will contain on the orderof 5 to g./l. sodium oxalate.

FIGURE 5 is a chart showing the precipitation results of test work usinguntreated pregnant liquor with washed and unwashed seed. In these teststhe results of seeding pregnant liquor with unwashed seed were comparedwith seeding pregnant liquor with washed seed. At the end of theprecipitation step (end of cycle 1 in FIG- URE 5) the total seed bed(seed plus product) was sampled and weighed. In the case of the unwashedseed there was essentially no growth of the alumina hydrate particlewhereas the growth in the case of the washed seed was very substantial;as indicated in FIGURE 5 the fines had decreased from about 80% to 8%.The spent liquor from the first cycle was then adjusted with bauxite tosimulate plant conditions and again used as pregnant liquor for thesecond cycle. The proportionate amount of seed from the firstprecipitation cycle was used as seed for the second cycle. Again, in thecase of the unwashed seed, no substantial amount of growth of thealumina hydrate particles were shown whereas the fines in the seed bedin the washed seed precipitate was reduced to 2%, which clearly showsthe efficacy of the instant invention in promoting growth of the aluminahydrate particles during precipitation.

It is to be understood that various changes and modifications may bemade in the foregoing method invention without departing from the spiritof the invention and the scope of the appended claims. Although theinvention is described in particular in regard to removal of oxalatefrom alumina hydrate seed particles, the invention should not beconstrued as limited thereto. The practice of the invention mayadditionally remove other substances that are also deleterious to themaking of product alumina.

We claim:

1. In the Bayer process wherein alumina is extracted from aluminousmaterial by digesting said aluminous material in a caustic liquor andsaid extracted alumina is precipitated from pregnant caustic liquorcontaining oxalate as a contaminant, which oxalate also precipitates andcontaminates the precipitated alumina hydrate, and the precipitatedsolids are separated from the major portion of the mother liquor afterwhich the concentrated solidscontaining slurry is classified accordingto particle size into product alumina hydrate and alumina hydrate seedfor recycle to the aforesaid precipitation, the improvement in particlesize control and increased capacity of precipitation comprisingseparating the spent caustic liquor from the alumina hydrate seed byfiltering and subjecting the alumina hydrate filter cake to cold waterwashing, the amount of cold water used being not in excess of that whichis sufiicient to remove substantially all of the spent caustic liquorremaining in the seed solids, recycling of the spent caustic liquor tothe digestion phase of the Bayer process, subjecting thecold-water-washed-alumina hydrate to a washing with hot water todissolve substantially all of the sodium oxalate contaminant, separatingthe washed seed from the oxalate-containing wash liquor, and thereafterrecycling the washed seed to the precipitation reaction.

2. The process of claim 1 wherein the oxalate-containing liquor istreated to reprecipitate oxalate.

3. The process of claim 1 wherein the oxalate-containing liquor istreated with caustic to reprecipitate sodium oxalate.

4. The process of claim 1 wherein the oxalate-containing liquor istreated with lime in sulficient quantity to causticize the sodiumoxalate to obtain recoverable caustic and insoluble calcium oxalate.

5. The process of claim 4 wherein the calcium oxalate is subjected tocalcination to regenerate the lime.

6. In the Bayer process wherein alumina is extracted from aluminousmaterial by digesting said aluminous material in a caustic liquor andsaid extracted alumina is precipitated from pregnant caustic liquorcontaining oxalate as a contaminant, which oxalate also precipitates andcontaminates the precipitated alumina hydrate, and the precipitatedsolids are separated from the major portion of the mother liquor afterwhich the concentrated solidscontaining slurry is classified accordingto particle size into product alumina hydrate and alumina hydrate seedfor recycle to the aforesaid precipitation, the improvement in particlesize control and increased capacity of precipitation comprisingseparating the spent caustic liquor from the alumina hydrate seed byfiltering and subjecting the alimina hydrate filter cake to cold waterwashing, said cold water temperature being not more than about 32 C. andthe amount of cold water used being not in excess of that which issuificient to remove substantially all of the spent caustic liquorremaining in the seed solids, recycling of the spent caustic liquor tothe digestion phase of the Bayer process, subjecting thecold-water-washed-alumina hydrate to a washing with hot water todissolve substantially all of the sodium oxalate contaminant, said hotWater being at a temperature from about 50 C. to 90 C., separating thewashed seed from the oxalate-containing wash liquor, and thereafterrecycling the washed seed to the precipitation reaction.

7. The process of claim 6 wherein the oxalate-containing liquor istreated to reprecipitate oxalate.

8. The process of claim '6 wherein the oxalate-containing liquor istreated with caustic to reprecipitate sodium oxalate.

9. The process of claim 6 wherein the oxalate-containing liquor istreated with lime in suflicient quantity to causticize the sodiumoxalate to obtain recoverable caustic and insoluble calcium oxalate.

10. The process of claim 9 wherein the calcium oxalate is subjected tocalcination to regenerate the lime.

References Cited UNITED STATES PATENTS 2,806,766 9/1957 Anderson 231432,935,376 5/1960 Roberts 23-143 MILTON WEISSMAN, Primary Examiner.

OSCAR R. VERTTZ, Examiner.

H. T. CARTER, Assistant Examiner.

1. IN THE BAYER PROCESS WHEREIN ALUMINA IS EXTRACTED FROM ALUMINOUSMATERIAL BY DIGESTING SAID ALUMINOUS MATERIAL IN A CAUSTIC LIQUOR ANDSAID ALUMINA IS PRECIPITATED FROM PREGNANT CAUSTIC LIQUOR CONTAININGOXALATE AS A CONTAMINANT, WHICH OXALATE ALSO PRECIPITATES ANDCONTAMINATES THE PRECIPITATED ALUMINA HYDRATE, AND THE PRECIPITATEDSOLIDS ARE SEPARATED FROM THE MAJOR PORTION OF THE MOTHER LIQUID AFTERWHICH THE CONCENTRATED SOLIDSCONTAINING SLURRY IS CLASSIFIED ACCORDINGTO PARTICLE SIZE INTO PRODUCT ALUMINA HYDRATE AND ALUMINA HYDRATE SEEDFOR RECYCLE TO THE AFORESAID PRECIPITATION, THE IMPROVEMENT IN PARTICLESIZE CONTROL AND INCREASED CAPACITY OF PRECIPITATION COMPRISINGSEPARATING THE SPENT CAUSTIC LIQUOR FROM THE ALUMINA HYDRATE SEED BYFILTERING AND SUBJECTING THE ALUMINA HYDRATE FILTER CAKE TO COLD WATERWASHING, THE AMOUNT OF COLD WATER USED BEING NOT IN EXCESS OF THAT WHICHIS SUFFICIENT TO REMOVE SUBSTANTIALLY ALL OF THE SPENT CAUSTIC LIQUORREMAINING IN THE SEED SOLIDS, RECYCLING OF THE SPENT CAUSTIC LIQUOR TOTHE DIGESTION PHASE OF THE BAYER PROCESS, SUBJECTING THECOLD-WATER-WASHED-ALUMINA HYDRATE TO A WASHING WITH HOT WATER TODISSOLVE SUBSTANTIALLY ALL OF THE SODIUM OXALATE CONTAMINANT, SEPARATINGTHE WASHED SEED FROM THE OXALATE-CONTAINING WASH LIQUOR, AND THEREAFTERRECYCLING THE WASHED SEED TO THE PRECIPITATION REACTION.